One well-known kind of photovoltaic device has two or more cells connected in series formed from a vertical layer structure of semiconductor materials, each cell containing a p-n junction of a different bandgap, which junctions are used to absorb different parts of the spectrum of the light falling on the device (Tandem Cell). The majority of multi-junction photovoltaic devices that are currently in use are manufactured on a germanium substrate.
FIG. 1 shows the typical structure of the lower cell in such a device. A p-type germanium (Ge) substrate 1 is provided and the first p-n junction 2 of the device is fabricated by growing a layer 3 of a III-V semiconductor material onto the germanium substrate, the two meeting at interface 8. This layer 3 is termed, in the art, a nucleation layer. At the elevated temperatures used during processing, Group V atoms from the nucleation layer 3 diffuse across the interface into the germanium substrate creating the p-n junction 2 at a point below the surface of the germanium. The junction forms because the Group V atoms act as an n-type dopant in the germanium and so when they have diffused in sufficient concentration an n-type region 4 is formed. (The other border of the n-type region 4 is of course the interface 8 between the III-V and the Group IV materials.) The Group III-V layer 3 is provided n-type so that there is low resistance contact between that and the Group IV n-type region 4. Control of the depth of the diffusion of the Group V atoms is important in defining the quality of the p-n junction, with a shallower junction being preferable. Diffusion is controlled by the temperature and duration of the growth and annealing (and any other processing) of both the nucleation layer 3 of any further semiconductor layers 5. The further layers 5 of semiconductor are provided to form one or more further p-n junctions for absorbing different parts of the spectrum. Examples of multi-junction solar cells having a bottom cell similar to FIG. 1 are given by U.S. Pat. No. 6,380,601 and US 2002/0040727.
A paper Si as a diffusion barrier in Ge/GaAs hetero junctions by S. Strite, M. S. Ünlü, K. Adomi, and H. Morkoc (Appl. Phys Lett. 56(17)) was published in 1990. The authors of this paper were interested in phototransistors and hole based modulation doped structures, and the paper itself discussed their investigation of a diode formed of a gallium arsenide epitaxial layer overgrown with germanium. This basic diode was said to suffer from microplasma assisted breakdown caused by poor sample uniformity caused, they suggest, by vacancies in the GaAs (caused in turn by Ga and As outdiffusion into the Ge). An interlayer of pseudomorphic silicon of 10 Å in thickness was provided to prevent this. (The germanium was p-type, being doped with Ga to a concentration of 5×1018 cm−3. The GaAs at the junction was more lightly doped, to a concentration of 5×1016 cm−3, with silicon.) In particular it may be controlled to a predetermined depth.
Solar cells are used to generate electrical power, preferably from sunlight. They may be used directly irradiated by the sun or with concentrators that gather sunlight onto the cell in a higher concentration, which improves their efficiency.